Natural water, represented by the chemical formula H.sub.2 O, normally contains a proportion of deuterium oxide HDO or D.sub.2 O in amounts which may be as low as, say, 143 parts per million. It is known that deuterium oxide, or so-called heavy water, is useful for many purposes particularly in the nuclear-reactor field where it forms a primary coolant or a reaction moderator and is required in large quantities for heavy-water nuclear reactors.
In the production of heavy water or in the recovery of deuterium, it is known to enrich naturally occurring water in order to improve the economy of any extraction process. As pointed out in my earlier applications mentioned above, it has already been proposed for the production of heavy water to utilize techniques which occur naturally in augmenting the deuterium-oxide concentration in natural water. In fact, naturally occurring water derived from large bodies, such as oceans, seas and lakes, which undergo a constant process of concentration as a consequence of evaporation, are well known as advantageous sources of deuterium oxide because the concentration therein is higher than the concentration of water in other sources which do not undergo evaporation as readily.
While it is desirable to make use of water having the highest natural concentration of deuterium oxide as a source of water for processes by which deuterium oxide can be recovered or deuterium can be removed from water, difficulties are encountered with the use of water from the larger naturally occurring bodies. For example, sea water is not an effective source because the high salt content introduces difficulties in operation and, because of scaling, requires equipment designed to resist corrosion and having a capacity for scale removal etc. If a prior salt-removal step is used to render the water more desirable for deuterium-oxide recovery, considerable expense is involved.
It has also been proposed to enrich water with deuterium oxide and tritium oxide by drawing upon chemical systems which use water in the process. For example, a so-called "parasitic" enrichment process has been suggested in which a predetermined quantity of water necessary for or used in another chemical process is employed as a source of water. The deuterium-oxide content is thereby increased by an isotope-exchange reaction between the hydrogen of the water and deuterium elsewhere in the chemical system. These processes are, however, limited by the amount of water which can be obtained from such chemical processes, a quantity which is relatively small, and by the limited amount of deuterium which will exchange for hydrogen under the conditions customary in such systems.
It should also be noted that a process of enrichment of the deuterium concentration occurs naturally in conventional electric-power-generating plants utilizing a cooling circuit in conjunction with the primary steam circuit. In such electric-power-generating systems, the heat from a boiler operated by combustible or fissionable fuels may convert water to steam (generally superheated) which is introduced under pressure to a steam turbine mechanically coupled to an electric generator. The expansion of steam in the turbine drives the rotor of the latter and, consequently, the generator. The depleted steam is then passed through the condenser in indirect heat exchange with the secondary coolant, i.e. water. The circulation path of the latter includes a pump and a cooling tower open to the atmosphere. Since a portion of the secondary cooling water is continuously evaporated into the atmosphere, a concentration of deuterium oxide occurs naturally in this secondary cooling in this secondary cooling circuit, albeit at a low rate. The condensate (primary liquid) is returned to the heat source while evaporation losses are compensated by make-up water generally containing only the deuterium-oxide concentration of natural water.